Limestone Coal Formation
Limestone Coal Formation is part of the Clackmannan Group
Forsyth et al. (1996) changed the name Limestone Coal Group to Limestone Coal Formation, but without changes to the boundaries.
The Limestone Coal Formation (see Browne et al., 1999, fig. 4) comprises sandstone, siltstone and mudstone, mostly in repeated upward-coarsening cycles, though some fine-upwards. The cycles are usually capped by seatearth and coal. The siltstone and mudstone are usually grey to black, while the sandstone is usually fine- to medium-grained and off-white to grey. Locally named coal seams are common and many exceed 0.3 m in thickness. Minor lithologies include cannel, and blackband and clayband ironstone, the latter nodular as well as bedded. Nonmarine limestone is rare and marine limestones are present only locally towards the bottom of the formation. Upward-fining parts of the succession, dominated by fine-grained to locally coarse-grained sandstone, are widely developed, and thick multistorey sandstones are present. Locally, successions may be particularly sandy or argillaceous. Unlike the Johnstone Shell Bed and ‘Black Metals Marine Bands’ that can be correlated widely, the coal seams are not so easily correlated and retain their local names.
Fluviodeltaic (‘Millstone Grit’) facies. The cycles indicate periodic delta progradation, with the ‘marine bands’ resulting from marine transgression.
Type sections are represented by two boreholes in north-east Glasgow; the upper part of the formation from 316.0 to 539.5 m depth in the Cardowan No. 2 Borehole (BGS Registration Number NS66NE/66) (NS 6706 6752) and the lower part from 0 to 106.7 m depth in the Cardowan No. 13 Borehole (BGS Registration Number NS66NE/104) (NS 6706 6875).
Lower and upper boundaries
The base of the formation is taken at the top of the Top Hosie Limestone (TOHO) of the Lower Limestone Formation, and the top is drawn at the base of the Index Limestone (ILS) of the Upper Limestone Formation (Figure 6, Column 4). The Lower and Upper limestone formations include conspicuous beds of laterally extensive limestone, with diverse marine faunas, whilst the Limestone Coal Formation, in contrast, has prominent beds of coal.
The maximum thickness of the formation is greater than 550 m in the Clackmannan area of the Central Coalfield (Browne et al., 1985, p. 11). In Ayrshire the sequence is condensed and less than 100 m thick. Generalised thicknesses were given of 110 m for the formation on Arran (BGS, 1987a) and 25–105 m in the main coalfield area at Machrihanish (BGS, 1996).
Distribution and regional correlation
Throughout the Midland Valley of Scotland, the Isle of Arran and at Machrihanish.
Age and biostratigraphical characterisation
Namurian (Pendleian). Beds containing large numbers of shells (coquinas) of Lingula, or of the nonmarine bivalves Naiadites and Curvirimula, occur in the fine-grained rocks (including ironstones and cannel). Because of the form of preservation, these shells usually do not form conspicuous ‘musselbands’ like those of the Scottish Coal Measures Group. Marine shells are present in some fine-grained strata, and the Johnstone Shell Bed and ‘Black Metals Marine Bands’ can be correlated throughout the Midland Valley.
The formation has yeilded no goniatites (Currie, 1954). The Johnstone Shell Bed is normally developed in two beds, the lower carrying the richer faunal assemblage dominated by the annelid Serpuloides carbonarius, the brachiopods Pleuropugnoides cf. pleurodon, Lingula squamiformis and Productus spp., the gastropods Euphemites spp. and Retispira spp., and the bivalves Anthraconeilo luciniformis, A. mansoni and Streblopteria ornata (Wilson, 1967). The ‘Black Metals Marine Bands’ yield macrofossils including annelids, brachiopods and bivalves (see below) in most parts of central Scotland, although in some areas only Lingula sp. is present. Note that amongst the productoid brachiopod genera Productus is dominant in the Johnstone Shell Bed and rare in the ‘Black Metals Marine Bands’, whilst Buxtonia dominates in the ‘Black Metals Marine Bands’ and is rare in the Johnstone Shell Bed. Non-marine bivalves are represented in the Limestone Coal Formation by species of Curvirimula, Naiadites and Paracarbonicola.
- Forsyth, I H, Hall, I H S, and McMillan, A A.1996.Geology of the Airdrie district.Memoir of the British Geological Survey, Sheet 31W (Scotland).
- Browne, M A E, Dean, M T, Hall, I H S, McAdam, A D, Monro, S K, and Chisholm, J I.1999.A lithostratigraphical framework for the Carboniferous rocks of the Midland Valley of Scotland.British Geological Survey Research Report, RR/99/07
- Browne, M A E, Hargreaves, R L, and Smith, I F.1985.The Upper Palaeozoic basins of the Midland Valley of Scotland.Investigation of the geothermal potential of the UK. (Keyworth, Nottingham: British Geological Survey.)
- British Geological Survey.1987a.Arran. Scotland Special Sheet, 1:50.000 Series. Bedrock. (Southampton: Ordnance Survey for the British Geological Survey.)
- British Geological Survey.1996.Campbeltown. Scotland Sheet 12, Provisional Series. Solid and Drift 1:50.000. (Keyworth, Nottingham: British Geological Survey.)
- Currie, E D.1954.Scottish Carboniferous goniatites.Transactions of the Royal Society of Edinburgh, Vol. 62, Pt. 2, 527–602
- Wilson, R B.1967.A study of some Namurian marine faunas of central Scotland.Transactions of the Royal Society of Edinburgh, Vol. 66, 445–490.